Why can a last-second 'Stop!' call still fail on a runway?

Answer: Reaction time is too short

Radio waves slow at night — Wrong. Radio waves do not become slower at night. The problem is not signal speed; it is that human reaction and vehicle stopping distance still take time, while the runway conflict is arriving almost immediately.

Jets erase the signal — Wrong. Jet blast is not what makes the warning ineffective here. The deeper issue is timing: once a high-speed landing aircraft and a vehicle are already too close, even a correct last-second warning may come after the safest escape options have nearly disappeared.

Reaction time is too short ✓ — Correct! By the final seconds, one or more earlier safety layers have already failed. Even if the call is heard immediately, there may not be enough distance left for a vehicle to stop or enough time left for the aircraft to avoid the conflict. The late warning is real, but the usable window may already be gone.